Based on the OAM-SM scheme, the maximum likelihood method is adopted to demodulate the radiated symbol after the transmission antenna index and the OAM state have been estimated. Hence, the bit error probability of the radiated symbol is derived by [1]}, [2]}
\(e_{{\text{mod}}}^{\text{b}} = \frac{1}{{P{{\log }_2}P}}\sum \limits _{p = 1}^P {\sum \limits _{{p_1} = 1}^P {\mathbb {D}({x_p},{x_{{p_1}}})} \mathbb {Q}\left( {\sqrt{\frac{\rho }{{2\sigma _w^2}}{{\left| {h_{mm}^l} \right|}^2}{{\left| {{x_p},{x_{{p_1}}}} \right|}^2}} } \right)} ,\qquad \mathrm {(38)}\) 
